Design aesthetic is one of the very important added-values of industrial products today. Accordingly, there are many products that have various kinds of texture and surface structures. For example, the outer coating materials of automobiles contain various kinds of effect materials and accordingly their surfaces have extremely minute glitter reflective and granular texture. Furthermore, colors significantly change according to variations in combinations of effect materials, pigments and other materials, layer structures of coating, and geometrical conditions of lighting and observation directions. In particular, the advent of interference effect materials in recent years has added variety to colors and texture. In addition to outer coating of automobiles, the surface structures of other products such as interior materials, furniture, buildings, electric appliances, electronic devices, cosmetics, and packages have become complex regardless of inclusion of effect materials. Like outer coating of automobiles, the colors and texture of products change in various ways depending on texture and optical geometrical conditions. Furthermore, wood products and textile products as well as coated products exhibit color changes and texture changes depending on optical geometrical conditions. Not only industrial products, but also skin and biological matters exhibit color changes and texture changes depending on optical geometrical conditions.
Product development of designs and materials, manufacturing and quality control, and marketing such as advertisements have required a high level of visual evaluation by experienced people and therefore have required time and human resource consuming and required huge energy and enormous efforts. There are demands for establishment of methods for accurately measuring and evaluating colors of surfaces of products and means for accurately representing images such as CGs.
Thus, it is desired to establish physical means and an evaluation method for accurately and stably measuring image color information at multiple angles with simple operations. Note that as color measurement, spectral measurement is essential especially for colors of objects containing interference effect materials because of its accuracy and capability of measuring a wide range of colors.
Patent document 1 discloses a technique that uses light dispersed by a prism or a diffraction grating to acquire spectral information that differs among parts of an image.
Patent document 2 discloses a technique relating to a multi-angle spectral imaging apparatus including a rotary illumination light source and a multispectral camera.
Patent document 3 discloses a technique in which a white reference surface is provided around an entire sample surface in order to correct time fluctuations in an illumination light source.
Patent document 4 discloses a technique that uses a lighting device including a plurality of monochromatic LEDs that are arranged in a row and capable of emitting light rays of different colors to illuminate a sample surface and take an image when multi-angle spectral measurement is performed.
Patent document 5 discloses a technique relating to a spectral measurement system including a spectrometer that measures a first spectrum of a subject under ultraviolet illumination light and a second spectrum of the subject under visible illumination light.
Patent document 6 discloses a technique relating to a color measurement apparatus that measures spectral characteristics of a sample and performs computations.